1. Field of the Invention
This invention relates to a primer composition used to make silicone type hard coats adhere strongly to various substrates, inparticular, transparent plastic substrates, and also relates to an article having a primer coating, obtained using the primer composition. More particularly, this invention relates to a primer composition containing a benzotriazole derivative having a small heat loss, contained as an ultraviolet light absorber having a superior anti-volatility when processed at a high-temperature and having a superior retention in a primer coat thin film formed.
2. Description of the Prior Art
Among various substrates, plastic substrates which are light-weight and have superior impact resistance and transparency have been used widely as substitutes for glass. They, however, have disadvantages such that they loose transparency easily because of their low surface hardness and tendency to be scratched, and also they become yellow outdoors easily because of their poor weatherability. Other substrates such as metals, ceramics and wood workpieces also have similar disadvantages more or less.
Accordingly, various methods are proposed in order to improve weatherability and scratch resistance (or wear resistance) of articles making use of such substrates.
In order to improve scratch resistance, it is attempted to treat a substrate surface with, e.g., a silicone type hard-coating material composed chiefly of a hydrolyzable silane and/or a partial hydrolysis-condensation product thereof, to cover it with a high-hardness protective film. In this method, when treated with the hard-coating material directly, the coat formed has a poor adhesion, and hence a primer coat of various types is provided before the protective coat is formed on the substrate. In many cases, ultraviolet light absorbers of various types are contained in such primer coats so that the weatherability of substrates can be improved.
Known ultraviolet light absorbers used generally for such purpose include salicylic acid type ultraviolet light absorbers such as phenyl salicylate, benzophenone type ultraviolet light absorbers such as hydroxybenzophenone and 2,4-dihydroxy-4-methoxybenzophenone, benzotriazole type ultraviolet light absorbers such as 2-(2'-hydroxy-5'-methylphenyl)benzotriazole and 2-(2'-hydroxy-3',5'-di-t-butylphenyl)benzotriazole, and cyanoacrylate type ultraviolet light absorbers such as 2-methylhexyl-2-cyano-3,3'-diphenyl acrylate and ethyl-2-cyano-3,3'-diphenyl acrylate. In particular, those put into wide and general use in view of a good solubility in various solvents and resins are 2-(2'-hydroxy-5'-t-butylphenyl)benzotriazole and 2,4-dihydroxybenzophenone. These, however, may become decomposed or volatilized when primers or coating materials are heated to cure, so that the ultraviolet light absorber can not be incorporated in a sufficient quantity into the coating formed finally, making it difficult to achieve a sufficient weatherability.
In practice, used as ultraviolet light absorbers are, e.g., benzylidene malonate compounds and/or cyanoacrylate compounds as disclosed in Japanese Pre-examination Patent Publication (kokai) No. 55-160033, benzophenone compounds and/or triazine compounds as disclosed in Japanese Pre-examination Patent Publication (kokai) No. 58-179237, and benzotriazole compounds as disclosed in Japanese Pre-examination Patent Publication (kokai) Nos. 1-149879 and 2-182764. All the ultraviolet light absorbers, however, have a relatively low molecular weight, and hence may become decomposed partly or volatilized at the time of heat curing when primer coats or protective coats are provided, resulting in an insufficient ultraviolet screening effect. Thus, they have not reached a satisfactory level in respect of weatherability.